Combine 2 cups softened vanilla light ice cream (to soften ice cream, transfer it to the refrigerator for about 30 minutes), 1 ounce bittersweet chocolate, coarsely chopped, and 1 ounce chopped (into 1/4-inch pieces) crystallized ginger in a bowl. Stir together with a rubber spatula or wooden spoon. Arrange 6 chocolate wafer cookies (such as Nabisco Famous Chocolate Wafers) or chocolate graham crackers (you'll need 12 total) on a baking sheet, and top each with about 1/4 cup of the ice cream mixture. Top ice cream with another wafer, lightly pressing to form a sandwich. Cover ice cream sandwiches with plastic wrap, and place in the freezer. Freeze sandwiches at least 2 hours before serving.